(1) Field of the Invention
This invention relates to a high Mn-Cr non-magnetic steel with good resistance to stress corrosion cracking (hereinafter referred to as "SCC" for brevity), and more particularly to a high Mn-Cr non-magnetic steel suitable for use as a material for retainer rings of power generator rotors.
(2) Description of Prior Art
Generally, the retainer rings for generator rotors are required to be non-magnetic to prevent drops in the efficiency of power generation. In addition, the material for the rotor retainer ring is required to meet demands for strength to cope with increases in the capacity of power generators. For example, in the case of an atomic power plant, a rotor with a winding assembly therearound is put in high speed rotation while feeding cooling water to conduits in rotor bearings to avoid overheating. The rotor is accommodated in a stator which is filled with hydrogen gas or air and contacted with cooling water to deprive of its radiation heat.
The cylindrical retainer rings which are fitted on the bearings at the opposite ends of the rotor are cooled in a similar manner. The retainer rings are usually formed of a non-magnetic material, mainly from 18Mn-5Cr steel from the standpoint of preventing drops in the efficiency of power generation by the production of eddy current. This kind of steel has a high strength but suffers from a problem in that it becomes susceptible to stress corrosion cracking during repeated use over a long time period. Although the cause of this problem has not yet been cleared completely, it has been confirmed by experiments that the resistance to stress corrosion cracking is lowered considerably by moisture deposition. In this connection, although the retainer rings are not cooled by direct contact with cooling water in the actual generator as mentioned above, there are possibilities of the hydrogen gas or moisture content in air condensing on the surfaces of the retainer rings when cooled off at the time of interruption of operation or of leaking water from the stator depositing on the surfaces of the retainer rings, causing a drop in resistance to stress corrosion cracking of the retainer rings.
In order to prevent the stress corrosion cracking of the retainer rings, there have been employed stress corrosion cracking resistant steel materials with various carbon contents like, for example, 12Mn-18Cr steel, which however all turned out to have a problem with regard to workability or strength.